A variety of power storage devices, for example, non-aqueous secondary batteries such as lithium ion batteries (LIBs), lithium ion capacitors (LICs), and air cells have been actively developed in recent years. In particular, demand for lithium ion batteries with high output and high energy density has rapidly grown with the development of the semiconductor industry, as in the cases of electrical appliances, for example, portable information terminals such as mobile phones, smartphones, and laptop computers, portable music players, and digital cameras; medical equipment; and next-generation clean energy vehicles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs). The lithium ion batteries are essential for today's information society as chargeable energy supply sources.
A negative electrode for the power storage devices such as the lithium ion batteries and the lithium ion capacitors is a structure body including at least a current collector (hereinafter referred to as a negative electrode current collector) and an active material layer (hereinafter referred to as a negative electrode active material layer) provided over a surface of the negative electrode current collector. The negative electrode active material layer contains an active material (hereinafter referred to as a negative electrode active material), such as carbon or silicon, which can store and release lithium ions serving as carrier ions.
At present, a negative electrode of a lithium ion battery using a graphite based carbon material is generally formed by mixing graphite (black lead) that is a negative electrode active material, acetylene black (AB) as a conductive additive, PVDF that is a resin as a binder to form slurry, applying the slurry over a current collector, and drying the slurry, for example.
Such a negative electrode of a lithium ion battery and a lithium ion capacitor has an extremely low electrode potential and a high reducing ability. For this reason, an electrolyte solution using an organic solvent is reduced and decomposed. The range of potentials in which the electrolysis of an electrolyte solution does not occur is referred to as a potential window. The negative electrode essentially needs to have an electrode potential in the potential window of the electrolyte solution. However, the negative electrode potential of a lithium ion battery or a lithium ion capacitor is out of the potential windows of almost all electrolyte solutions. Actually, a decomposition product thereof forms a passivating film (also referred to as a solid electrolyte film or solid electrolyte interphase (SEI)) on the surface of the negative electrode, and the passivating film prevents further reductive decomposition. Consequently, lithium ions can be inserted into the negative electrode with the use of a low electrode potential below the potential window of the electrolyte solution (e.g., Non-Patent Document 1).
However, such a film on the surface of the negative electrode which is formed by the decomposition product of the electrolyte solution kinetically suppresses the decomposition of the electrolyte solution, which leads to a gradual deterioration. Therefore, it cannot be said that such a film is a stable film. Since the decomposition reaction particularly speeds up at high temperature, the decomposition reaction greatly hinders operation of the negative electrode in high temperature environments. In addition, the formation of the film on the surface generates irreversible capacity, so that part of discharge capacity is lost. For these reasons, there are demands for a film on the surface of the negative electrode which is more stable and can be formed without losing its capacity.